Cook Sausage: Fry the sausage in a skillet until it's browned. Stir in the cheddar cheese sauce and set the mixture aside.

Prepare Bread Base: Spread the bread cubes evenly in a 10x6 baking dish. Top the bread with the browned sausage and shredded Mexican cheese.

Whisk Egg Mixture: In a large mixing bowl, whisk together the eggs, mustard, salt, and pepper until thoroughly combined. Pour in the milk and continue whisking until well incorporated.

Assemble Casserole: Pour the prepared egg mixture over the assembled casserole, ensuring even coverage. If desired, you can cover the dish and refrigerate it overnight at this stage.

Bake: Preheat the oven to 400°F. Bake the casserole for 1 hour. Then, uncover it and continue baking for an additional 15-20 minutes until the casserole has puffed up and is no longer wet in the center.

Serve: Once baked, take the casserole out of the oven, let it cool slightly, and serve. Enjoy your delicious sausage and cheese breakfast casserole!